FIRST, check with your pediatrician to make sure it’s safe to exercise. If he/she clears you to workout and you are under 14, then I recommend working out using body weight exercises like pushups and pullups, sit ups, and running until you are a little older. Once you are 14, you can add in weights, resistance bands and other types of training. As long as you eat to maintain a healthy weight, your growth will NOT be stunted. Do not take supplements outside of medicines a doctor prescribed to you personally and a multivitamin. See if your gym or fitness center offers an orientation workout to show you how to get started. Most gyms offer this free to new customers as a one time gift. Some facilities even offer fitness programs for teens, so check into that option as well. Good Luck!